The present invention is directed to a process for the preparation of an organic electroluminescent device; and, more particularly, to a process for the preparation of an organic electroluminescent device containing a polyimide thin layer having an improved thermal stability, surface uniformity, and high density by way of vapor deposition polymerization.
Generally, an organic electroluminescent device has a laminated structure comprising a transparent electrode layer, a metallic electrode layer, and an organic interlayer including an organic luminescent layer arranged between the two electrodes. The organic electroluminescent device can be operated with an alternating current(AC) or direct current(DC) power supply. In case of DC operation, the transparent electrode functions as an anode, and the metallic electrode as a cathode. In order to increase the luminance efficiency, the organic interlayer may further include hole transport agents and electron transport agents, often in a multilayer configuration.
For example, in case of DC operation, a separate hole transport layer may be disposed between, and in close contact with, the anode layer and one surface of the organic luminescent layer. Further, an optional electron transport layer may be placed between the cathode layer and the organic luminescent layer. Depending on the organic materials employed, therefore, the organic interlayer of an organic electroluminescent device may be in the form of single, double or triple layers, each layer containing various combinations of electronically active materials, i.e., organic luminescent materials, hole transport agents and electron transport agents. As the anode layer, indium tin oxide-glass layer is usually used while a metallic layer of magnesium, aluminum, indium or silver-magnesium can be used as the cathode.
In conventional organic electroluminescent devices, the organic layers are usually formed by vapor-depositing an active material such as a hole transport agent and organic luminescent material. However, an organic layer, e.g., a hole transport layer, prepared by a conventional vapor deposition method of an electronically active material, e.g., a hole transport material, has disadvantages in that the deposited organic layer is fragile and easily broken by a vibrational shock and the active material is often recrystallized because of the low glass transition temperature of the active material. For example, the glass transition temperature of N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-ethylphenyl)-1,1xe2x80x2-diphenyl-4,4xe2x80x2-diamine, one of the hole transport agents is about 63xc2x0 C. Further, the lifetime of the device may be shortened due to the occurrence of recrystallization and diffusion migration phenomena during its use.
In order to solve the above problems, an organic layer has been prepared by dispersing an active material in a polymer matrix by a wet process such as spin coating using an organic solution thereof. However, the organic layer thus obtained has the deficiencies of surface roughness, low bulk density and contamination by residual organic solvent. Further, when the organic interlayer is of a multiple layer form, the interlayer adhesion is not sufficiently high, leading to a deteriorated interfacial contact and decreasing lifetime of the device. Moreover, it is difficult to accurately control the thickness of an organic layer by a conventional wet process and the reproducibility of the process is low, which may, in turn, result in poor performance of the device.
Therefore, there has existed a need to develop a process for the preparation of an organic electroluminescent device having an improved interfacial surface roughness, high bulk density, thermal stability and improved interfacial contact.
Accordingly, it is a primary object of the present invention to provide a process for preparing an organic electroluminescent device containing an organic interlayer having an improved surface roughness, high bulk density, thermal stability and improved interfacial contact.
In accordance with the present invention, there is provided a process for preparing an organic electroluminescent device having a transparent substrate, a transparent electrode layer, a metallic electrode layer, and an organic interlayer disposed between and in close contact with the electrode layers, the organic interlayer being comprised of an organic luminescent layer, an optional hole transport layer and an optional electron transport layer and containing an electronically active material dispersed in a matrix of polyimide of formula (I), characterized in that the organic interlayer is prepared by: i) depositing the vapors of a dianhydride, a diamine and the electronically active material to form a polyimide precursor layer containing the active material dispersed therein; and ii) thermally imidizing the polyimide precursor layer: 
wherein A is derived from a dianhydride; B is derived from a diamine; and n is an integer of 2 or higher.